Starch is a mainstay of our food, pharmaceutical, paper, chemical, textile, cosmetic, and energy industries. It serves as a raw material in the manufacture of such basic consumer necessities as paper and textiles, and is also used in sizing, surface coating and adhesives. Corn starches and their derivatives are also used in a variety of other applications, including the manufacture of “drilling muds” that are used to cool oil well drilling bits, and in flocculating agents, anti-caking agents, mold-release agents, dusting powder and thickening agents. A developing market for corn starch is for use as a feedstock for production of a number of industrial chemicals and plastics that heretofore relied on petroleum-derived feedstocks. As the world's petroleum supply dwindles or becomes less reliable, the importance of an abundant renewable natural source of raw material such as corn starch becomes increasingly attractive.
The wet milling process is traditionally used to separate corn into its primary components of starch, germ, fiber and protein. The wet milling processes, for example, generally comprise four basic stages: steeping, germ separation, grinding/screening, and starch-gluten separation. After inspection and cleaning, corn kernels are steeped in a dilute aqueous SO2 solution for 1 to 2 days to soften the corn kernels and begin breaking disulfide bonds in the protein matrix that holds the proteins and starch together. The next step in the process involves a coarse grind to separate the germ from the rest of the kernel. The resulting slurry, consisting of fiber, starch and protein, is finely ground and screened to separate the fiber from the starch and protein. The starch is separated from the remaining slurry in hydrocyclones. The starch can then can be used for making drilling mud, or can be further processed for a variety of other applications.
The steeping operation is a diffusion limited process. During the 24-48 hour soak, the water and SO2 diffuse into the corn kernel through the base end of the tip cap, and then move through the pericarp to the kernel crown and into the endosperm. The absorbed SO2 cleaves the disulfide bonds in the protein matrix that encapsulates the starch granules, dispersing the protein matrix, and enhancing starch release. The time for penetration of SO2 into the endosperm and its reaction time with the protein matrix makes steeping a very time consuming operation in the corn wet-milling process. Steeping times shorter than 24 hours result in poor starch yields and loss of starch to fiber and protein fractions. Steeping is also one of the most capital and energy intensive parts of the corn wet-milling process. Reducing steep time would decrease energy cost, increase plant capacity and reduce the capital cost involved in construction of new corn wet-milling plants.
Several mechanical and chemical approaches have been investigated to decrease steep time while maintaining product yields. Such processes typically require costly modifications of existing facilities or pretreatment of kernels, resulting in increased pollution or increased energy use. It has been said that the development of a processing procedure that could reduce the steep time and decrease or eliminate the use of chemicals such as sulfur dioxide would have a significant impact on the corn wet-milling industry.
Processes in which dehulled and degermed dry-milled corn is mixed with water for a period of not more than 4 hours at ambient temperature to form an aqueous slurry have also been explored. In some cases, the slurry is treated with alkali, and then subjected to certain high intensity mixing to yield a high-quality starch. Sodium sulfite may be present in the alkali-containing slurry. Existing processes and production facilities for producing starch from corn are typically subject to various constraints including mass flow limitations, product yield, plant size and energy consumption. Accordingly, there is continued interest in the development of ways to improve the selectivity and yield of starch from corn.